1. Field of the Invention
The present invention relates to a polishing end point detection method, and more particularly to an optical polishing end point detection method for detecting an end point of a polishing operation (e.g., a polishing stop point or a changing point of polishing conditions) of a film of a substrate.
2. Description of the Related Art
In a semiconductor fabrication process, a CVD apparatus is widely used for forming an oxide film on a wafer. In this film formation process, wafers are processed on a lot basis. Typically, twenty-five wafers are processed as one lot. In one lot, a thickness of the oxide film does not greatly vary between wafers. However, when several tens of lots are processed, the thickness of the oxide film may vary within plus or minus 10% due to a change with time in consumable member or temperature in a film-forming apparatus.
Further, in a case where the film-forming apparatus has multiple chambers for the film-forming operations, the thickness of the film can also vary between the chambers. It is not particularly problematic when the deposited film is thin. However, when the film is thick, an initial thickness thereof can greatly vary between the wafers. For example, when the film is made to have an initial thickness of 1800 nm, the initial thickness may vary within plus or minus 180 nm. Such variations in the initial thickness can present a problem of an error in polishing end point detection.
An optical polishing end point detection method is generally used for detecting the polishing end point of the oxide film. An example of the optical polishing end point detection method will be described below with reference to FIG. 1. In the optical polishing end point detection method, light is applied to a surface of a film during polishing of the film, and a characteristic value is calculated using an intensity of the reflected light (i.e., a reflection intensity). This characteristic value is calculated by dividing a reflection intensity at a predetermined wavelength by a reflection intensity at another predetermined wavelength. The characteristic value is a value obtained through a process of removing noise components from the reflection intensity. This characteristic value changes periodically according to a change in the film thickness. This is because of interference between a light ray reflected off the surface of the film and a light ray reflected off a surface of an underlying layer underneath the film.
A horizontal axis in FIG. 1 indicates a polishing time (or a film thickness), and a vertical axis indicates the characteristic value. As shown in FIG. 1, as the polishing process progresses, the characteristic value changes in a sine-curve pattern. At an initial stage of the polishing process, a waveform of the characteristic value may be unstable due to an unstable polishing phase and noises. Therefore, a start point of monitoring the characteristic value is intentionally delayed, so that monitoring of the characteristic value is not started until the polishing process progresses to a certain degree.
After monitoring of the characteristic value is started, the number of local maximum points (or local minimum points) is counted. When a predetermined number of local maximum points appear, the polishing operation is terminated. In FIG. 1, a fourth local maximum point V is preset as the polishing end point. Therefore, when the count of the local maximum points reaches four after monitoring is started, the polishing operation is terminated.
FIG. 2 is a graph showing a characteristic value in a case of polishing a film with a large initial thickness as compared with the film in the case of FIG. 1. Due to the large initial film thickness, a first local maximum point I is shifted to appear after the monitoring start point. Consequently, a fourth local maximum point IV is determined to be a polishing end point. As a result, a local maximum point V, which is an original polishing end point, is not detected, and this results in insufficient polishing of the film. On the other hand, when the film has a small initial thickness, a local maximum point II is shifted to appear before the monitoring start point. Consequently, a third local maximum point V is not determined to be a polishing end point, and this results in excessive polishing of the film. An amount of such shift of the local maximum point is also affected by a polishing rate. Therefore, in order to avoid an error in the polishing end point detection, it is necessary to measure the initial thickness and the polishing rate beforehand even if wafers have an identical device structure and to alter a recipe for the polishing end point detection based on the measurement results. These procedures make management of the polishing operations complicated and lower productivity.